1. Field of the Invention
The present invention concerns a device and a method for transmission of magnetic resonance signals from a number of local coils to an evaluation device, of the type wherein the signals are transferred as optical signals in an optical conductor between a first, transmitter-side transducer that transduces electrical signals into optical signals and a second, receiver-side transducer that transduces the optical signals into electrical signals, an optical conductor that conducts the optical signals, and wherein an analog/digital converter is provided between the local coils and the first, transmitter-side transducer.
2. Description of the Prior Art
Standard electrical conductors (for the most part coaxial cables) are used in conventional magnetic resonance systems in order to feed the magnetic resonance signals acquired by the local coils attached on the body of a patient to an evaluation device. Due to the radio-frequency alternating magnetic field predominating in the examination volume, currents known as sheath waves or common mode currents that exhibit the same frequency as the alternating magnetic field are induced in the electrical conductors. Without further measures, the common mode currents also flow into the human body due to the capacitive coupling with the patient. This must be avoided since internal burns can occur due to a current flow into the human body. This problem was previously solved by the components known as sheath wave barriers that are attached to the electrical conductors between the local coils and the evaluation device. A sheath wave barrier is formed by a parallel resonance circuit with a specific inductance L and a specific capacitance C connected in parallel. L and C must be tuned such that resonance occurs in the oscillating circuit at the frequency of the alternating magnetic field (and thus the frequency of the current induced in the electrical conductors). In the case of resonance in which the total resistance of the oscillating circuit is very high, the oscillating circuit (which inductively couples to the electrical conductor) maximally attenuates the current induced in the electrical conductor, which is why a sheath current no longer occurs on the electrical conductor and the human body (which is capacitively coupled with the electrical conductor); the sheath waves are thus suppressed. Since a number of such sheath wave barriers are required for the multiple electrical feed lines (one cable is required per local coil), the device is relatively complex, occupies a large volume and is costly. This known device is described in DE 102004015856 A1, for example.
Another approach to solve the problem of the currents induced in the electrical conductors by the alternating magnetic field is described by G. P. Koste, M. C. Nielsen, T. R. Tolliver, R. L. Frey, R. D. Watkins, GE Global Research Center, “Optical MR Receive Coil Array Interconnect”, Proceedings ISMRM 2005, p. 411. The signal transmission in electrical conductors is replaced by optical signal transmission in optical fibers. Analog optical signals are transferred between the measurement unit (essentially formed by local coil and amplifier) and the evaluation unit that comprises a receiver that transduces the optical signals back into electrical signals. In the measurement unit, the transduction of electrical signals into optical signals occurs in a modulator (for example a Mach-Zehnder optical modulator made from LiNbO3) into which laser light and electrical measurement signals are fed. The information from the signals of the local coils is thereby modulated to the laser light by intensity modulation. The modulated light is then analogously transferred to the receiver. Furthermore, it is important that LiNbO3 is suitable for use in high magnetic fields. A disadvantage of such analog optical signal transfer with a modulator is that the signal/noise ratio is relatively poor, which is why the dynamic range (thus the capability of the system to differentiate between large and small signals) is limited. LiNbO3 modulators are additionally relatively expensive.
Yet a further approach is to digitize the analog electrical signals of the local coils before the transfer to the evaluation device and to transduce the digital electrical signals into digital optical signals. This method is disclosed WO 2006/008665 A1. The signal/noise ratio is optimized by the early digitizing of the signals; influences by the strong magnetic fields are eliminated by the transfer as optical signals. A suitable electronic device with a transducer for transduction of electrical signals into optical signals is provided for each local coil.
As noted above, the solution with sheath wave barriers on the electrical conductors is thus that this is technically complicated and expensive.
Although the solution of analog optical transfer in connection with optical modulators avoids the problem of the sheath waves, the achievable signal/noise ratio is not satisfactory and the financial expenditure is relatively high.
A problem with the solution with digital optical transmission as described in WO 2006/008665 A1 is that a separate readout electronic each one electro-optical transducer must be provided for each local coil, which entails a relatively large space requirement and is costly.